Raf-induced cell cycle progression in human TF-1 hematopoietic cells.

Ras/Raf/MEK/ERK is a crucial pathway regulating cell cycle progression, apoptosis, and drug resistance. The Ras oncogene is frequently mutated in human cancer, which can result in the activation of the downstream Raf/MEK/ERK cascade leading to cell cycle progression in the absence of a growth stimulus. Raf-induced proliferation has been observed in hematopoietic cells. However, the mechanisms by which Raf affects cell cycle progression are not well described. To investigate the importance of Raf/MEK/ERK signaling in human hematopoietic cell growth, the effects of three different Raf genes, A-Raf, B-Raf and Raf-1, on cell cycle progression and regulatory gene expression were examined in TF-1 cells transformed to grow in response to beta-estradiol-regulated DeltaRaf:ER genes. Raf activation increased the expression of cyclin A, cyclin D, cyclin E, and p21(Cip1), which are associated with G(1) progression. Activated DeltaRaf-1:ER and DeltaA-Raf:ER but not DeltaB-Raf:ER increased Cdk2 and Cdk4 kinase activity. The regulatory role of p16(Ink4a), a potent Cdk4 kinase inhibitor, on the kinase activity of Cdk2 and Cdk4 was also examined. Raf induced p16(Ink4a) suppressor but this did not eliminate Cdk4 kinase activity. These results indicate that human hematopoietic cells transformed to grow in response to activated Raf can be used to elucidate the mechanisms by which various cell cycle regulatory molecules effect cell cycle progression. Furthermore, the differences that the various Raf isoforms have on Cdk4 activity and other cell cycle regulatory molecules can be determined in these cells.